Mr Johnson said in response to Sir Kim's resignation on Wednesday: "My view is it's wrong to drag civil servants into the political arena."

Announcing his resignation in a letter, Sir Kim wrote: "There has been a great deal of speculation surrounding my position and the duration of my remaining term as ambassador. I want to put an end to that speculation.

"The current situation is making it impossible for me to carry out my role as I would like.

"Although my posting is not due to end until the end of this year, I believe in the current circumstances the responsible course is to allow the appointment of a new ambassador."

Boris under fire

The departure comes after Mr Johnson was asked in a television debate whether Sir Kim would retain his job under his government. He said it was not "necessarily" the right thing for Trump to have said, but added: "Let's face it, our relationship with the U.S. is of fantastic importance."

Sir Alan Duncan, who worked with Mr Johnson in the Foreign Office, has laid into his former boss. He told the BBC: "I'm upset and angry. "There are a lot of people here in the Commons who are very, very angry and feel he has lost so much respect for having done what he's done.

"His sort of disregard for Sir Kim Darroch and his refusal to back him was, in my view, pretty contemptible, but also not in the interests of the country he's trying to lead."

"For someone who wants to lead, that was contemptible negligence on his part and he's basically thrown this diplomat under the bus to serve his own personal interests".

Labour deputy leader Tom Watson said: "This is a personal tragedy for Kim Darroch. It is the inevitable result of a leak organised by dark forces who do down our country, and Boris Johnson's failure to stand up for our sovereignty. Always personal ambition before country with Johnson."

Scotland's First Minister Nicola Sturgeon tweeted: "It is shameful that Kim Darroch has effectively been forced out for doing the job that diplomats are appointed to do. Boris Johnson's failure last night to stand up for him – and stand up to the behaviour of Donald Trump – spoke volumes."

Brexit Party leader Nigel Farage has described the resignation of Sir Kim Darroch from the position of UK ambassador to Washington as "The right decision."

He tweeted: "Time (to) put in a non-Remainer who wants a trade deal with America."

A 'very stupid guy'

In his secret memos dating from 2017 to the present, Darroch had said reports of in-fighting in the White House were "mostly true" and last month described confusion within the administration over Trump's decision to call off a military strike on Iran.

"We don't really believe this Administration is going to become substantially more normal; less dysfunctional; less unpredictable; less faction riven; less diplomatically clumsy and inept," Darroch wrote in one cable.

The comments sparked an immediate backlash from the President, who lashed out at "foolish" Theresa May and her "wacky" Washington ambassador on Tuesday.

"The wacky Ambassador that the UK foisted upon the United States is not someone we are thrilled with, a very stupid guy," Trump wrote, describing Darroch as a "pompous fool".

The wacky Ambassador that the U.K. foisted upon the United States is not someone we are thrilled with, a very stupid guy. He should speak to his country, and Prime Minister May, about their failed Brexit negotiation, and not be upset with my criticism of how badly it was...

...handled. I told @theresa_may how to do that deal, but she went her own foolish way-was unable to get it done. A disaster! I don't know the Ambassador but have been told he is a pompous fool. Tell him the USA now has the best Economy & Military anywhere in the World, by far...